This project will use the technique of electron probe microanalysis, coupled with the ancillary techniques of transmission electron microscopy and low temperature oxygen plasma microincineration to investigate calcium transport in individual cells of the chick chorioallantoic membrane, and the small intestine of rat and chick, epithelia which actively transport this element. The same techniques will be employed to establish whether a morphological blood-bone barrier exists, and if so, whether there is evidence for a cellular control of this barrier. The feasibility of using these techniques to study the fluxes of calcium to and from intracellular compartments and calcium pools in cells which carry out operations sensitive to intracellular calcium concentration will also be investigated.